The declaration and definition of shared_ptr::shared_ptr(nullptr_t), given in 19.11.3 [util.smartptr.shared], is

constexpr shared_ptr(nullptr_t) : shared_ptr() { }

The intention seems clear: this constructor should have the same semantics of the default constructor. However, contrarily to the
default constructor, this one is not noexcept. In contrast, unique_ptr::unique_ptr(nullptr_t) is noexcept,
as per 19.11.1.2 [unique.ptr.single]:

constexpr unique_ptr(nullptr_t) noexcept : unique_ptr() { }

Both libstdc++ and libc++ have added noexcept to shared_ptr::shared_ptr(nullptr_t). Microsoft's STL has not.

[2014-03-26 Library reflector vote]

The issue has been identified as Tentatively Ready based on six votes in favour.